This invention relates generally to electrolytic cells and more particularly to the protection of the inner wall surface of the heads of such cells from corrosion.
Electrolytic reactions are utilized in industry to produce various gases, for example, chlorine and hydrogen. Such reactions are commonly carried out in a device known as an electrolytic cell. An understanding of the problems associated with the operation of such cells can best be grasped by referring to one such cell commonly used in industry, the chlorine electrolytic cell.
Chlorine electrolytic cells are used in the production of chlorine and hydrogen gases. These cells commonly comprise an upper, generally dome shaped head connected to an underlying brine tank.
In operation, an electrolyte comprising a brine solution of sodium chloride (NaCl) and water injected into the cell and retained in the brine tank and head is subjected to an electric current passed between a cathode and an anode. The current causes chloride ions to migrate to the anode where the chlorine gas (Cl.sub.2) is formed. Hydrogen gas (H.sub.2) and liquid sodium hydroxide (NaOH) are also formed. This particular electrolytic reaction is expressed as: EQU 2NaCl+2H.sub.2 O.fwdarw.H.sub.2 +Cl.sub.2 +2NaOH.
As the process proceeds, bubbles of chlorine and hydrogen gases rise through the liquid and collect in the upper portion of the cell head above the liquid surface. As the reaction continues, the level of brine in the cell drops. The reaction is carried out as a batch process, with the hydrogen and chlorine gases being taken off the top of the cell head and the sodium hydroxide remaining as a liquid in the brine tank.
Corrosion of electrolytic cells can be a problem depending on the reactivity of the reaction products with the material of which the cell is made. For example, the cell head of the chlorine cell is commonly made of fiberglass reinforced plastic (FRP), while the brine tank is commonly made of concrete. The fiberglass and chlorine gas in combination are reactive.
As chlorine production occurs in such a cell, a butter-like material approximately 1/8 inch thick coats the inside wall surfaces of the cell head. This "chlorine butter" is a reaction product formed by a chlorine contacting the FRP walls of the cell head. The butter coating is beneficial in that it protects the walls of the cell head from further reaction with the chlorine gas which would otherwise eventually destroy the walls from corrosion.
Chlorine bubbles rising through the liquid in the cell during electrolysis scour the cell head walls, removing the protective chlorine butter coating and enabling further reaction and corrosion. Thus, eventually the cell head corrodes to a point that it must be rebuilt or replaced to prevent it from developing cracks or holes which would allow the escape of deadly chlorine gas. Typically, this rebuilding or replacement must take place every one-and-one-half to two years. Both the rebuilding and replacement process are expensive and time consuming. There is, therefore, a need for improved means to protect cell heads against corrosion, thereby to increase their life and the safety of their use, and to reduce the frequency and cost of their replacement or repair.
The primary objective of the present invention is to inhibit the corrosion of electrolytic cell heads, thereby to increase their useful life and the safety of their use.